It is sometimes advantageous to coat a substrate, especially a metal, with a coating having special properties, for example, wear resistance or corrosion resistance. Wear resistance can be provided for a substrate by coating it with a coating composed of a mixture of hard abrasive material such as powdered tungsten carbide or the like, dispersed in a hard filler metal. Corrosion resistance can be provided to a substrate with a similar coating composed of a corrosion resistant metal or alloy. Other special or improved physical properties can be obtained in a similar manner substituting other matrix materials.
Prior to the work of Breton et al. (U. S. Pat. Nos. 3,743,556, 3,916,506, and 4,194,040 the specifications and claims of which are incorporated herein by reference) it was difficult to produce such a coating, especially on objects having an intricate or complicated shape or requiring a coating thickness of greater than 0.020 of an inch. In the prior methods using plasma and flame spraying techniques, it was difficult to get uniform coatings on a substrate, especially a substrate having an intricate surface. A similar difficulty occurred in the prior method using techniques such as abrasive particle dusting onto the substrate.
Breton et al eliminated many of the difficulties that existed in the prior art methods by providing a method using a first layer of a desired thickness of a high melting point powdered matrix material in an organic binder material and a second layer of a lower melting temperature powdered brazing filler material also in an organic binder. The first layer is then placed on the substrate with the second layer in turn placed on top of it. The matrix material is characterized as being wetted by the brazing filler metal or alloy in the molten state. This assembly is then heated to decompose the binder and melt the filler metal or alloy which is infused by capillary action into the matrix layer. Cooling then yields an essentially void-free coating bonded on the substrate. The method for preparing both the hard particle matrix material as well as the braze filler alloy layers or performs, using fibrillated polytretra fluoroethylene (PTFE) is disclosed in U. S. Pat. Nos.3,916,506 and 4,194,040.
The coating typically consists of hard particles such as tungsten carbide, chromium carbide, titanium carbide, nickel boride or diamond etc. and provides a very hard and brittle surface, which is susceptible to cracking under thermal or mechanical stresses. As the flexible preform coatings are generally brazed in a controlled atmosphere or vacuum furnace at a temperature above the liquids temperature of the braze filler alloy and the cooling rate is generally slow it causes grain growth in the metallic substrate, which results in loss in hardness as well as the strength. In many instances, the coated parts are later heat treated to achieve required hardness and the strength in the substrate. Depending upon the substrate alloys and required properties, the heat treating conditions vary, but in most common alloy steels (e.g., AISI 1045, 4130, 4140 , 4340 and some tool steel) it is required that the parts be heated in excess of 1500.degree. F. and hardened by quenching in forced air, molten salt bath, oil or water. Quenching a coated part in any of the stated media may induce extreme thermal stresses in the coating and may result in cracking. These cracks may continue to grow from coating to substrate while being heat treated or later from mechanical stresses imposed during usage. Crack growth in the substrate will lead to premature failure of the coated article or component.
In conventional coating methods, e.g. , weld overlay, spraying, fused or plasma coating processes., etc., a softer metallic coating is sometimes applied on the substrate surface prior to applying the hardface coating. The reason for applying this softer layer between hardface coating and substrate is that it acts as a barrier to crack growth into the substrate thus preventing a catastrophic failure during the usage. In common practice, the soft intermediate layer is applied in a separate step by a weld overlay or a spray process.
The disadvantage with weld overlay method is that the surface of the layer is very rough and non-uniform in thickness, therefore, machining or grinding of the surface is required prior to hardfacing in addition to the fact that it requires a separate manufacturing step. Also, this process is not feasible where the surface to be coated is out of line of sight or has very complex geometry. The spray method may be a better process for such application since it provides a more uniform thickness of soft layer, with a smoother surface, however, it still requires the additional manufacturing step and where the area to be coated is out of the line of sight or on an inner surface, such as a small diameter pipe, this method cannot provide consistent durable results.